Put complex shape flat on the table
For simple situations you can probably get away with snapping or the drop to ground addon. 

For more complex situations, rigid body physics can be used:



You can probably use the convex hull collision shape (also see unexpected reactions in a quiet simple rigidbody physics simulation), but if you need concave collisions then you'll have to use the Mesh collision shape.

The mesh collision shape can be quite unstable, mesh to mesh collisions in particular.

One way to mitigate this is by increase the number of simulation steps in Properties > Scene > Rigid Body World:



But this doesn't always work completely. If you need to use the mesh collision shape, try to have it collide with a non-mesh shape if possible.

Also see Tips for making Mesh Rigid-body collision shapes more stable